In recent years, in a series regulator on an electronic control unit for automotive, a large-capacity electrolytic capacitor has been required for the purpose of supplying an electric power to a microcomputer during a period when a flash-ROM mounted microcomputer (microcomputer) for controlling an in-vehicle device writes information required for a subsequent start in the flash-ROM after a power supply from an in-vehicle power supply (battery, etc.) has been cut off. An event where the power supply from the in-vehicle power supply is cut off occurs in a rapid drop in a voltage of the in-vehicle power supply when a load large in power consumption starts, a disconnection, or a case where a relay controlled according to the operation of an ignition switch turns off for some reasons.
When it is detected that the operating voltage of the microcomputer drops due to a relay off or the disconnection, the microcomputer shifts to a standby state after a period required for writing in the flash-ROM. There is a need to continuously supply a given current to the microcomputer with a given operating voltage until the microcomputer shifts to the standby state. For that reason, measures are taken to increase the capacitance of the capacitor connected to an input terminal of the microcomputer.
In order to supply an operating current 400 [mA] to the microcomputer with a voltage change (ΔV=0.3[V]) of an operating voltage lower limit 3.0[V] from a normal operating voltage 3.3[V] of the microcomputer during a period of 140 [us] until the microcomputer shifts to the standby state, a large capacitance of C=400 [mA]÷0.3[V]×[us]=180 [uF] or more has been required. In order to realize this large capacitance, a large-sized electrolytic capacitor or a secondary battery has been required. The former is inept in downsizing, and the latter suffers from an increase in the costs.
As a method for solving the above problem, there is proposed a method disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2008-289254. In the method disclosed in the publication, an external voltage is monitored to detect a voltage drop with the results that, as compared with a case in which a microcomputer voltage is monitored, a reset response of the microcomputer is improved to avoid an indefinite operation of the microcomputer without any increase in the capacitance of the capacitor.